1. Technical Field
Embodiments of the present invention relate generally to satellite-based navigation systems and, more particularly, but not exclusively, to a method of generating a database useful in enhancing the accuracy of such systems in some situations.
2. Description of Related Art
In recent years military and civilian navigation have benefited from the advent of space-based global navigation satellite systems (GNSS). Currently, GNSS systems in active use include the GPS system operated by the U.S.A. and the GLONASS system operated by Russia. In a GNSS system a constellation of satellites in orbit around the Earth transmit signals that are picked up and analyzed by terrestrial receivers to determine a user's position. The receivers may include, for example, handheld units used by pedestrians and military infantry, and on-board receivers installed in automobiles, other motor vehicles, boats, and aircraft.
In order to reach every point on Earth, GNSS systems typically require a minimum of 24 satellites to be in orbit at any one time. However, for security, maintenance, and enhanced accuracy purposes additional satellites may be deployed. The GPS system, for example, has approximately 32 satellites in orbit. At any given time some of the additional satellites may be offline for maintenance, with others are actively transmitting. The number of satellite transmission signals received by a GNSS receiver at any point in time is determined by the receiver's location, and more particularly by the presence or absence of obstacles in the satellite path. For example, a receiver situated in an open plain or on a boat in open sea might receive as many as 12 satellite signals, whereas a receiver located in a valley may only receive half as many.
A user's position in space may be defined by three parameters: latitude, longitude, and elevation. Latitude and longitude may be characterized as horizontal coordinates “x” and “y”, and elevation as vertical coordinate “z”. Since the clocks used for timing by the satellites and the receivers are not synchronized, a fourth parameter “t”, representing the time bias between the user's receiver and the satellite system clock is also required. Accordingly, signal transmissions from four satellites are required to determine the user's position accurately. The four signals provide the mathematical basis for four equations, from which the four unknowns x, y, z, and t, may be solved. The internal calculations are commonly performed using a satellite-oriented coordinate system, and are transformed by the receiver into more familiar latitude, longitude, and elevation figures for the user's convenient reference. This type of solution, in which four satellite signals are used to determine a user's full position in space, is referred to as a 3-D solution.
If a receiver is positioned so that it only receives signals from three satellites, it is still possible to solve the navigation equations by assuming that the user's elevation is known. In such cases the receiver only needs to solve the equations for the three unknowns, x, y, and t, so three received signals are sufficient. Since the user's elevation is not solved, this type of solution is usually referred to as a 2-D solution.
In situations where more than four satellite signals are received, the receiver may employ various algorithms, such as better estimation equations or multipath mitigation techniques, to obtain a solution that is even more accurate than a solution based on four satellite signals.
One of the most difficult situations for modern GNSS receivers occurs when they are used in urban areas such as cities or towns, and particularly in dense urban environments such as the downtown core of major cities. These environments, sometimes known as “urban canyons”, are characterized by the presence of many tall buildings. This often creates two significant negative effects on GNSS receiver performance.
A first issue is that the high buildings may simply block the line of sight between the receiver and a transmitting satellite, thereby decreasing the number of satellites visible to the receiver. This may lead, for example, to a less accurate 2-D solution from three satellite measurements instead of a more accurate 3-D solution with four signals or more.
Another issue is that some of the signals that are received may have been reflected along the way from one or more adjacent buildings, travelling from satellite to receiver along a multipath rather than a single, direct path. This issue is exacerbated by the fact that the glass and metal exteriors of modern buildings generally act as reflectors of incident electromagnetic radiation. Furthermore, it is difficult for receivers to identify multipath signals and to distinguish them from direct path signals.
Multipath signals are a problem because their path length is longer than a true line-of-sight signal from satellite to receiver. Due to the fact that the positioning equations solved by the satellite navigation receiver assume a direct line-of-sight between the satellite and the receiver, inaccurate user positioning will result. Satellite receivers do employ multipath mitigation techniques in order to combat the effects of multipath. These techniques usually result in a more complex receiver design.
In the absence of a solution to these and other problems related to urban use of GNSS receivers, the growth opportunities of some aspects of satellite navigation technology may be reduced.